This invention relates to an apparatus for growing zeolite crystals from an aqueous solution, by coupling microwave energy into the solution under pressure.
Zeolites are inorganic crystals with a pore or duct structure which is open toward the outside, and which permits versatile industrial applications. Zeolites are used, for example, as catalysts in the petrochemical industry, as additions to detergents, and as molecular sieves. Also, high-power sensors and microfilters are produced from zeolite crystals, and they are used as ion exchangers in applications for a more efficient purification of water and for the disposal of radioactive waste.
The conventional method for the production of zeolites in the lab is crystallization from an aqueous solution. For the optimal formation of zeolite crystals in this manner, it is necessary to heat the volume of the solution rapidly (within seconds) and locally in a uniform manner. The solution is held for up to 30 seconds at a temperature of 180.degree. C. and is then cooled again to the ambient temperature. In order to avoid boiling of the solution at 180.degree. C., a pressure of approximately 15 bar is required in the cell.
The requirement for a rapid and uniform heating of the aqueous zeolite solution can be implemented only by the use of microwaves. Conventionally heated resistance furnaces, for example, require hours for the heating. Furthermore, only the exterior edge of the specimen is heated directly; heat transport into the interior takes place with a certain time delay by way of thermal convection and heat conduction.
Crystallization by means of microwave heating was developed and tested only a few months ago. First promising results are available. (I. Girnus, et al., Adv.Mater. 1995, 7, No. 8, p. 711; A. Arafat, et al., ZEOLITES, 1993, Vol. 13, p. 162 ). In both cases, a pulsed MW system with mode mixing was used; nothing was reported concerning the homogeneity of the temperature distribution. In the case of Girnus, et al., the crystallization takes place without the admission of pressure. In the case of Arafat, et al., a container which is provided with a pressure piston and in which the zeolite solution was situated was arranged in a conventional microwave oven.
A first measurement of the dielectric constant .epsilon.' and .epsilon." of a zeolite A-solution at room temperature, for a frequency of 2.45 GHz, resulted in the following values: ##EQU1##
In comparison to pure water, a loss factor is therefore obtained which is higher by a factor of 15; that is, the zeolite solution acts like an electrically poorly conducting material. The direct beaming-in of microwaves therefore causes only a heating of the exterior edge and no volumetric heating of the solution. The requirement of a uniform heating can therefore not be met by means of this process.
In the field of the heating of fluids, it is known (German Patent Document DE 41 02 233 A1; U.S. Pat. Document U.S. 38 12 315 ; Stuwe: "Conventional and Dielectric Heating of Honey", in Elektrowarme International, No. 8, 1968, P. 284 to 291) that uniform heating can be achieved if the fluid is subjected to a high-frequency electric field, or an electric field situated in the microwave range which exists between two capacitor plates. In this case, the heating takes place by the action of pressure on the fluid.
It is an object of the present invention to provide an apparatus for growing zeolite crystals from an aqueous solution under an increased pressure and a raised temperature, which permits uniform heating of the whole volume of the solution to be heated, and which requires minimal space.
This object is achieved by the apparatus according to the invention, which comprises a metallic pressure tank for admitting pressure to the aqueous solution, and devices for feeding microwave energy (for example, 2.45 GHz) into the aqueous solution by means of a capacitor between whose plates the aqueous solution to be heated is situated. One of the capacitor plates is formed by a wall of the pressure tank, while the other is arranged in its interior. The electric field distribution in a capacitor is homogeneous with the exception of edge effects, and while the entry of the zeolite solution causes a change of the capacity, it does not introduce field distortion.
By integration of the capacitor and the pressure tank according to the invention, an extremely compact and easily operable apparatus is obtained.
The apparatus according to the invention is suitable for growing zeolite crystals on board a spacecraft or on earth.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.